Piezoelectric micromachined ultrasonic transducers (PMUTs) have been described in several earlier publications, including S. Shelton, et al, “CMOS-compatible AlN piezoelectric micromachined ultrasonic transducers,” 2009 IEEE International Ultrasonics Symposium, pp. 402-405, Rome, Italy, Sep. 20-32, 2009, incorporated by reference herein in its entirety. A typical PMUT is a multilayer membrane structure that is excited into flexural vibration using piezoelectric actuation. The membrane structure is often formed by etching through a silicon wafer to remove the material beneath the membrane, thereby allowing it to vibrate. This etch forms a hollow tube beneath the back-side of the membrane. Sound is emitted from the tube when the membrane vibrates and the tube may be designed as an acoustic resonator to improve acoustic performance of the PMUT, as described in S. Shelton, O. Rozen, A. Guedes, R. Przybyla, B. Boser, and D. A. Horsley, “Improved acoustic coupling of air-coupled micromachined ultrasonic transducers,” 27th IEEE International Conference on Micro Electro Mechanical Systems (MEMS 2014), pp. 753-756, San Francisco, Calif. 2014, incorporated by reference herein in its entirety. Packaging the PMUT chip is challenging for a number of reasons. First, the front side of the membrane must be free to vibrate and cannot be contacted or coated by another material. Second, sound emitted from the front side of the membrane will reflect off surfaces facing the membrane; these reflections must be controlled to avoid reducing the acoustic output from the tube on the membrane back side. Third, the wafer is often thinned before etching the acoustic tube. As a result, the final die is thin and perforated with an array of holes, increasing the sensitivity to packaging stress and increasing cross-talk between neighboring PMUTs on the die.
In U.S. Pat. Nos. 7,449,821 and 8,710,717, Dausch describes PMUT devices composed of a first PMUT substrate bonded to a second redistribution or IC substrate. In both devices, the bottom side of the PMUT substrate is bonded to the top of the IC substrate. Many complicated fabrication steps are required to realize through-wafer vias that connect the front-side PMUTs to the back-side electrical contacts. Moreover, the vibrating PMUTs are relatively far from the bonded interface, resulting in increased mechanical coupling between PMUTs (a problem that Dausch tries to address by adding an additional polymer isolation layer between PMUTs).
Accordingly, what is needed is a PMUT design and fabrication method that would overcome the above-identified issues. The design and the fabrication method should be easy to implement, cost-effective, and utilize existing chip and wafer assembly technology. It is within this context that aspects of the present disclosure arise.